Wavelength-division multiplexing transmission devices are configured, at receivers thereof, to split wavelength-division multiplexed light per wavelength which is input to the device, and to demodulate each of the split light of the wavelengths into original signals. Those devices are also configured, at transmitters thereof, to modulate a plurality of signals into light of different wavelengths and to perform wavelength-division multiplexing on the light of the wavelengths for transmission.
Some of those wavelength-division multiplexing transmission devices are equipped with a pass-through function. The pass-through function is implemented in a certain station to transfer wavelength-division multiplexed light input by a transmission path toward a next station, where the light to be transferred does not terminate at the certain station.
Description is given specifically of an operation of the wavelength-division multiplexing transmission device having the pass-through function. In a receiver side of the device, an optical amplifier optically amplifies wavelength-division multiplexed light input from a transmission channel-a, and outputs it to a 1:2 brancher. The optical amplifier fulfills a function of supplementing a transmission loss. The 1:2 brancher demultiplexes the wavelength-division multiplexed light into two parts of light. While one of the two parts of light is output to a wavelength separator, another one of the two parts of light is output (i.e. passed through) to a wavelength filter of a transmitter 2. The wavelength separator separates the wavelength-division multiplexed light from the 1:2 brancher into light of wavelengths per wavelength, and outputs them to a wavelength converter. The wavelength converter demodulates each of the separated light to generate original signals, and outputs those signals to a signal receiver as reception data.
In a transmitter side of the wavelength-division multiplexing transmission device, a wavelength converter modulates transmission data input from a signal transmitter per signal to generate light of wavelengths, and outputs them to a wavelength-division multiplexer. The wavelength-division multiplexer performs wavelength-division multiplexing on the light of the wavelengths from the wavelength converter, and output the multiplexed light to a 2:1 multiplexer.
Meanwhile, the wavelength filter receives the wavelength-division multiplexed light which has been passed through by the receiver. The wavelength filter blocks a part of the wavelength-division multiplexed light that should be terminated at the device, whereas passes other part of the light through to the 2:1 multiplexer.
The 2:1 multiplexer multiplexes the wavelength-division multiplexed light from the wavelength-division multiplexer and the wavelength-division multiplexed light from the wavelength filter, and outputs the multiplexed light to an optical amplifier. The optical amplifier amplifies the wavelength-division multiplexed light, and transmits the amplified light toward a wavelength-division multiplexing transmission device of a next station through a transmission line-b.
By the way, the transmitter is required to stabilize output level of the optical amplifier in order to prevent a receiver of the next station from deterioration of signals. However, in case where a trouble occurs in the above-mentioned transmission line-a of the wavelength-division multiplexing transmission device having the pass-through function, the output level of the optical amplifier may not be stabilized.
More specifically, if an input of wavelength-division multiplexed light to the receiver is interrupted due to the trouble in the transmission line-a, light of a wavelength based on transmission data is generated in the wavelength converter of the transmitter, whereas wavelength-division multiplexed light to be passed through does not exist. In this situation, the output level of the optical amplifier may vary from a normal value, and then a quality of transmission line may deteriorate.
In view of the foregoing circumstances, some attempts have been made in such a way that a configuration is provided to stabilize output of the optical amplifier by controlling the amplification rate of the optical amplifier in accordance with the number of wavelengths to be input to the optical amplifier and the output of the optical amplifier at the time (See Patent Literatures 1 to 3 in a citation list).